Red on yellow, kill a fellow. Red on black, friend of Jack.

That folk rhyme is supposed to help people distinguish venomous coral snakes from several non-venomous "mimics," animals that discourage predators by deceptively imitating a dangerous species.

Problem is, the rhyme is unreliable due to the vast amount of color and pattern variation, called color polymorphism, found in both coral snakes and their mimics. The harmless ground snake, a common coral snake mimic, displays four strikingly different color patterns, only one of which closely resembles its dangerous red-and-black-and-yellow-banded counterpart.

If a mimicry system offers protection from predators, then why hasn't evolution eliminated the "failed mimics," such as ground snakes sporting color patterns that don't remotely resemble a coral snake? That's the puzzle that U-M evolutionary biologist Alison Davis Rabosky has spent the last four years trying to solve.

Rabosky is an assistant research scientist in the Department of Ecology and Evolutionary Biology and an assistant curator of herpetology at the Museum of Zoology who worked with Christian L. Cox of the University of Virginia. They don't claim to have fully resolved the paradox, but they did gain insights that help explain the persistence of non-mimic color patterns in ground snakes, especially rare patterns. It turns out that if you're a ground snake, displaying a rare color pattern also provides an evolutionary edge. Their findings were published online June 26, 2013 in The American Naturalist.

Logic predicts that non-mimics should by eaten preferentially by predators and, given enough time, you should end up with a single color type in the population. So the widespread co-occurrence of mimic and non-mimic color patterns is a puzzling and longstanding evolutionary paradox," said Rabosky.

"Basically the predators get a search image for the most common morph in that population, and that's the one they'll hit until that morph becomes rare. Then the predators switch to whatever is now the most common morph," Rabosky said.

U-M News Service press release